Humming in the Pipes

Hi,

I have Boilermate 2000 and a Glowworm Micro 70FF. I added some Wickes inhibitor to the top tank, thought it would slowly circulate into the system over a year or two.

Now I'm sometimes getting a deep humming sound from the pipes (like a deep note on an organ) with some vibration, which seems to be linked to the hot tap. Even had it this morning when I'd left the boiler switched off overnight.

Any ideas pls?
Regards,
Ian

Think/hope i've solved this, after a bit more time on google! It was just a toilet cistern, which I'll have to replace but have turned down the input tap on it for now.

http://www.askmehelpdesk.com/plumbing/loud-fog-horn-noise-when-toilet-flushed-35192.html
http://answers.yahoo.com/question/index?qid=20070827122336AAP56hr

It did have me worried for a bit & of course turning on either the hot or the cold tap reduced the noise with this system.
Cheers,
Ian